Scientists have found the perfect balance of exercise and diet to achieve one of the most elusive fitness feats—gaining muscle while simultaneously losing fat.

It can happen fast, too. But we’re warning you now; it’s not easy and actually not highly recommended.

In the study, published in The American Journal of Clinical Nutrition, 40 young male participants (averaging 216lb) endured a month of hard exercise (resistance training and high-intensity interval training six days a week) and a low-calorie diet. All the men cut their calorie consumption by 40 percent, but they were divided into two subject diet groups: a high-protein (236g) group and a low-protein (118g) group. They consumed both animal and plant-based protein as well as milk-based whey protein beverages.

“It was a gruelling affair,” study author Stuart Phillips said in a press release. “These guys were in rough shape, but that was part of the plan. We wanted to see how quickly we could get them into shape: lose some fat, but still retain their muscle and improve their strength and fitness,” he added.

At the end of the four weeks, the high-protein group experienced about 2.5 pounds of muscle gains, despite consuming a fairly insufficient amount of calories (in order to drop weight). The lower protein group, on the other hand, didn’t add muscle; though they didn’t lose any muscle either. What’s more, the high-protein group also lost about 10.5 pounds, while the low-protein group lost just eight pounds.

Here’s why: “Exercise, particularly lifting weights, provides a signal for muscle to be retained even when you’re in a big calorie deficit,” said Phillips.

That being said, this is a pretty extreme weight-loss regimen. “We designed this program for overweight young men, although I’m sure it would work for young women too, to get fitter, stronger, and to lose weight fast. It’s a tough program and not something that’s sustainable or for those looking for quick and easy fix,” said Phillips. “We controlled their diets, we supervised the exercise, and we really kept these guys under our ‘scientific’ thumb for the four weeks the participants were in the study.”

An easier, more sustainable approach is in the works, the researchers say. So stay tuned!